Dooklear

A couple of days ago my 10 speed trans got up to 235 degrees while in a drive-thru. Only sat there about 5-10 min. I always monitor the temps and it was only 199 degrees when I pulled of the freeway. I drove about a mile and then waited in the drive-thru and it hit 235. While at a light today, it jumped from 201 to 214 almost instantly. However, the minute I start driving, the temp goes back to normal. Any thoughts? Yes I will take it to the dealer as it is still under warranty, but they want to keep it for 35 days just to diagnose. I can't be down 35 days as this is my work truck. Any thoughts? Also I can tow a 5000lb trailer and the issue is the same, no worse. As long as I am driving everything is fine.